May 6, 2005
Source: British Society for Plant Pathology, New Disease Reports, Vol. 11 [edited]
<http://www.bspp.org.uk/ndr/july2005/2005-32.asp>

Iris Yellow Spot Virus in onions: a new tospovirus record from India
K.S. Ravi, Division of Molecular Virology, Mahyco Research Center, Jalna-Aurangabad, Road, PO Box 76, Dawalwadi, JALNA 431 203, Maharashtra, India; A.S. Kitkaru (as for Ravil); and S. Winter, DSMZ Plant Virus Collection, Messeweg 11/12, 38104 Braunschweig, Germany Accepted for publication 11 Apr 2005

In the Jalna and Nasik regions of Maharashtra during 2002-03, plants of field-infected onion (_Allium cepa_) exhibited characteristic symptoms of chlorotic spindle or diamond-shaped lesions on leaves and scapes, with
twisting or bending flower-bearing stalks. In the advanced stages, single spindle-shaped chlorotic lesions coalesced, leading to withering of leaves and flower-bearing stalks.

The disease was transmitted to a number of virus indicator plants by mechanical inoculation using phosphate buffer (0.01 M sodium sulfate, pH 7.0, containing 0.1 percent sodium sulfite). Inoculated _Nicotiana benthamiana_ produced systemic necrotic lesions, eventually resulting in die- back and wilting of plants, while _N. tabacum_ (varieties 'Xanthi NC', 'White Burley', 'Samsun' and 'GT-4') and _N. clevelandii_ produced local chlorotic ring spots, 3 to 6 days after inoculation. In _Vigna unguiculata_ necrotic local lesions developed 3-4 days post inoculation. _N. rustica_ failed to produce any symptoms and were not infected.

Field-infected onion samples and greenhouse-inoculated plants were tested by ELISA, using the following antisera: Tobacco streak virus (DSMZ AS-0615), Watermelon silver mottle virus (DSMZ AS-0118), Iris Yellow Spot Virus (DSMZ AS-0528), Tomato Spotted Wilt Virus (DSMZ AS-0105), Impatiens Necrotic Spot Virus (DSMZ AS-0115), Chrysanthemum Stem Necrosis Virus (DSMZ AS-0529), Peanut Bud Necrosis Virus (ICRISAT) and Potato Virus Y (DSMZ AS-0573); utilizing the respective positive control samples.

Field-infected onion samples and the respective mechanically inoculated tobacco plants reacted strongly with IYSV antisera, but failed to react with any of the other antisera tested. RT-PCR using primers designed to the capsid gene and flanking sequences of IYSV (GenBank Acc. No. AF001387) produced the expected 925-bp amplicon from infected but not healthy onions.

Results of symptomatology, host range, ELISA and RT-PCR indicate that the causal virus is a strain of Iris Yellow Spot Virus, a new report from an important onion growing region of Maharashtra.

IYSV has been reported as a potentially devastating pathogen of onion in Europe (Cortes et al. 1998), Israel (Gera et al. 1998) and the United States (Gent et al. 2004).

References

Cortes I, Livieratos IC, Derks A, Peters D, Kormelink R, 1998. Molecular and serological characterization of Iris Yellow Spot Virus, a new and distinct tospovirus species. Phytopathology 88, 1276-1282.

Gent DH, Schwartz HF, Khosla R, 2004. Distribution and incidence of Iris Yellow Spot Virus in Colorado and its relation to onion plant population and yield. Plant Disease 88, 446-452.

Gera A, Cohen J, Salomon R, Raccah B, 1998. Iris Yellow Spot tospovirus detected in onion (Allium cepa) in Israel. Plant Disease 82, 127.

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[IYSV, a tospovirus, is difficult to readily identify. Research results to date have shown: i) disease incidence varies among varieties; ii) IYSV may be associated with plant stress (i.e. moisture, temperature extremes, salinity, soil compaction, pink root, etc. ); iii) in more susceptible varieties, IYSV incidence is initially higher along field edges and lower in the center of the field (similar to the pattern of onion thrips). This pattern of thrips distribution occurs when they immigrate from surrounding vegetation, but their distribution may become more uniform in the field as
the season progresses. Entomologists at Cornell University are studying the ecology of onion thrips and their movement patterns within and between fields; and iv) IYSV incidence decreases as plant population increases,
possibly because onion thrips are challenged to locate a single plant when plant populations are high. In a IYSV pesticide trial, it was found that Actigard + imidacloprid resulted in a 34-38 percent yield increase of the jumbo class. A complete integrated approach will be necessary for successful IYSV and thrips management. - Mod.DH

Links:
<http://www.eppo.org/QUARANTINE/Alert_List/viruses/irysxx.htm>
<http://vegetablemdonline.ppath.cornell.edu/NewsArticles/Onion_IYSV.htm>
<http://www.dpvweb.net/dpv/showadpv.php?dpvno=363#members>]